The aim of the proposed project is to investigate the premise that the induction of immunity to allogeneic tumor cells and the subsequent immune destruction of these cells are affected by separate subpopulations of thymus cells. The induction of immunity will be assayed by measuring mixed allogeneic cell responses. The cytotoxicity of the effector lymphocytes generated from this induction will be assayed by measuring release of 51 Chromium from labelled allogeneic tumor cells. For these cultures, subpopulations of thymus cells will be selected by a two-step procedure. First, by density gradient centrifugation, the light density fraction of thymus cells which is known to be responsive to mitogens will be selected. Second, cells responding to specific mitogens will be selectively removed from this first population by allowing responding cells to incorporate bromodeoxyuridine and then exposing them to light, thus inducing "suicide" in the responding subpopulation. Dead and damaged cells will be removed by a second density gradient centrifugation. Cell populations characterized in this manner will then be assayed to determine the efficacy of the "suicide" selection procedure by mitogen restimulation. Other assays on these same characterized populations will determine whether cell-mediated immune function as measured by alloantigen recognition and allogeneic target cell destruction requires one of the mitogen sensitive populations alone or several in combination to mount cell-mediated immune responses.